The present invention generally relates to binary data and more particularly, to a method of encoding and decoding the binary data at the time of recording and reproduction of the binary data and to a frame synchronous signal applied at the time of gathering of the encoded binary data into a frame.
Conventionally, in apparatuses for recording and reproducing binary data through conversion of analog signals such as acoustic signals, video signals, etc. into digital signals, there have been employed several methods of encoding and decoding the binary data for the purpose of recording and reproducing the binary data at a high density and stably. In the known methods, it has been generally so arranged that the binary data are divided into a plurality of groups each having a proper number of bits such that the groups of the binary data are converted, for encoding thereof, into another binary signals. For example, such a technique is disclosed in U.S. Pat. Nos. 3,624,637 and 3,641,525 in which 4-bit data are converted, for encoding thereof, into 5-bit codes, or in the so-called "3PM" method proposed by G. V. Jacoby (IEEE) Transactions on Magnetics Vol. MAG-13, No. 5, Sep 1977, P1202). Meanwhile, by generalizing such methods, P. A. Franaszek proposed a run length limited (RLL) code (IBM Journal of Res. & Dev., Vol. 14, July 1970, P376) in which m-bit groups of the binary data are converted into n-bit codes (m&lt;n) such that a run length of bits "0" generated in the converted codes in restricted to d to k in number. In order to obtain a desired density of recording and reproduction from such methods of encoding and decoding the binary data, performances of encoding and decoding are required to be examined.
Generally, in the case of recording on magnetic recording media, performances of encoding methods are evaluated mainly based on a minimum interval between transition Tmin, a maximum magnetizing transition interval Tmax, and a detection window width Tw required for identifying the magnetizing transition interval. A wave form of magnetic recording and reproduction is represented as a superposing of a reproduced wave form corresponding to the magnetizing transition. In the case where the minimum magnetizing transition interval Tmin is reduced so as to enable recording and reproduction at a high density, mutual interference of reproduced signals of the magnetizing transition read by a reproducing element such as a magnetic head, etc. increases, so that peak values or amplitudes of the reproduced signals vary enormously, thereby resulting in large errors of the detection. Accordingly, if the recording density is set at a predetermined value, mutual interference of the reproduced wave forms decreases as the minimum magnetizing transition interval Tmin is increased. This indicates that, in apparatuses of an identical wave form, it becomes possible to improve the recording density by employing an encoding method having a large minimum magnetizing transition interval Tmin. Namely, it will be understood that an encoding method having a larger minimum magnetizing transition interval Tmin is suitable for a higher recording density.
Meanwhile, in the case where the detection window width Tw required for identifying the magnetizing transition interval is large, a permissible range of shift in peak position (peak shift) due to mutual interference of the reproduced signals is wider, so that errors due to noises of the apparatuses, noises of the media, etc. occur less frequently. Furthermore, clocking is derived from the reproduced data. In the case where a period of the reproduced clock signal is larger than the maximum magnetizing transition interval Tmax, it becomes rather difficult to derive the clocking accurately.
Consequently, it can be concluded that such performances as a larger minimum magnetizing transition interval Tmin, a larger detection window width Tw and a smaller maximum magnetizing transition interval Tmax are desirable for encoding methods applied to apparatuses for recording and reproducing data at a high density. Then, a value of Tw.times.Tmin will be considered as a criterion for selection of an encoding method suitable for a higher density, hereinbelow. As shown in the column "Tw.times.Tmin" of Table 1, the modulating method "NRZ.NRZI" has a largest value of 1 and is followed by the modulating methods "3PM", "2/4M", "4/8NRZI", "HDM-1" and "HDM-2" each having a value of 0.75. However, the modulating method "NRZ.NRZI" has such a disadvantage that, since its maximum magnetizing transition interval Tmax undesirably assumes infinity, it is difficult to reproduce the clocking. Accordingly, the modulating methods "3PM", "2/4M", "4/8NRZI" and "HDM-1" have been generally employed as the encoding methods of magnetic tapes, magnetic disk apparatuses, etc. However, because of recent increase in amount of information, there is a strong demand for encoding methods suited for a higher density. When the value of Tw.times.Tmin is considered as the criterion for selecting an encoding method satisfying such a demand, it is desirable that the detection window width Tw and the minimum magnetizing transition interval Tmin be increased in balance with each other. Thus, the modulating method "HDM- 3" having the minimum magnetizing transition interval Tmin of 2T (T=bit period of data) in Table 1 may be regarded as a unique encoding method. However, since the modulating method "HDM-3" has the detection window width Tw of 0.33T, the value of Tw.times.Tmin is 0.67, so that the modulating method "HDM-3" is inferior to the modulating method "3PM" whose value of Tw.times.Tmin is 0.75.
Although the methods of encoding and decoding the binary data have been so far described, a plurality of coded words are, as a matter of fact, assembled into a frame without encoding the binary data and recording the code sequences as they are. The frame is provided with a frame synchronous signal for identifying the frame. At the time of reproduction, the frame synchronous signal is detected from the reproduced signal sequence and is used for controlling a clock phase or a starting point of grouping the codes at the time of the decoding. Accordingly, since the above described functions of the frame synchronous signal are important, the frame synchronous signal should be detected securely.
Conventionally, in the case of selection of synchronous signals, there have been usually employed bit strings in which erroneous detection results least frequently even if a bit deviation occurs, or special repeated bit strings having, for example, a large magnetizing transition interval. However, such known bit strings have such an inconvenience that the same bit strings exist in the encoded code sequences, thereby resulting in a strong possibility of erroneous detection of the synchronous signals. Accordingly, in order to prevent erroneous detection of the synchronous signals, a powerful synchronous protective circuit based on a phenomenon that synchronous signals are generated periodically have been required to be provided in the known bit strings. In addition, the known bit strings have been disadvantageous in that, since they are based on periodical generation of the synchronous signals, a time period in which the synchronous signals are securely detected after a long interval of absence of the synchronous signals becomes large. Thus, since the synchronous protection circuit is required to be adjusted in accordance with performances of recording and reproducing apparatuses, special attention is needed for the design and such a drawback was encountered that the circuit becomes complicated.